1. Field of the Invention
The present invention relates to an inverted telephoto type wide angle lens system having a relatively high aperture ratio of about 1/2.8 with a wide field angle of greater than 100 degrees.
2. Description of Prior Art
Attempts to increase the field of view of camera objectives while still providing images with relatively low distortion are well known. Experiences in the prior art with extra wide angle objectives of an inverted telephoto type have disclosed that negative distortion frequently increases as the photographing angle increases. This negative distortion further increases with an increase in the back focal length and in the aperture ratio, with the result that the aperture of the front lens tends to be undesirably enlarged.
The prior art has attempted to utilize a front lens group of a negative refractive power for attaining the necessary back focal distance to permit the movement of the reflex mirror. The reflex mirror is generally mounted to be swingably moved in an upward direction when an exposure is to be made in a single lens reflex camera. The use of a negative front lens group, however, generates considerable negative distortion and astigmatism and the correction of these aberrations are particularly difficult when a lens system is desired to have a greater field angle and a greater aperture ratio than the commercial wide angle lens systems usually available.
Various inverted telephoto type wide angle lens systems have been suggested in the prior art such as those disclosed in U.S. Pat. Nos. 3,832,035, 3,856,386, 3,877,795, 3,884,556, and 3,981,563. Additionally, Japanese Patent Application Laid Open Nos. 52-66422 and 53-55119 are cited of general interest.
The consumer requirements of the camera industry have placed increasing demands on lens designers to provide ever increasing optical performances while further compacting and making lightweight lens systems. Accordingly, even though the prior art is crowded in attempts to achieve improved optical performance, there is still a demand to provide an improved inverted telephoto type wide angle lens system.